In devices having contact surfaces for conducting electricity between two elements equipped with such contact surfaces (referred to herein as “electrical contacts”), an electrical continuity problem is well known in the art. As used herein, “electrical contact” should be taken to mean any surface which electrical current enters or exits, or apparatus having such a surface. Examples of such an apparatus are batteries, lamps, sliding switches as in a battery-powered flashlight, electrical relays, header connectors, circuit board terminals, computer peripheral back planes, and both solid-oxide and proton exchange membrane fuel cell assemblies. Inexpensive base metals typically used to form prior art electrical contacts, for example, aluminum, zinc, copper, tin, or stainless steel, either corrode or form electrically-resistive oxide layers on the surface of the contacts because of electrochemical activity and/or atmospheric attack at these surfaces. The non-conductive oxide surface layers increase the resistance of the metallic interface and inversely decrease the conductivity. Practical conduction of a surface in any application depends, of course, on the applied voltage and amperage; but for many applications, for example, in fuel cell assemblies, a measured resistance of greater than about 0.0015 ohmcm2 (Ωcm2) is unacceptable. In the prior art, electrical contacts requiring high conductivity and long durability are known to be coated with noble metals such as gold and platinum to prevent corrosion of the electrical contact surfaces, but such coatings are so expensive as to be impractical for many ordinary applications.
One example pertains to the electric contact resistance of hydrogen/oxygen fuel cells, wherein such considerations are especially relevant. In a proton exchange membrane (PEM) fuel cell stack, individual membrane electrode assemblies (MEA) are connected in electrical series using “bipolar plates” between the MEAs. The functions of each bipolar plate include connecting the individual fuel cells, distributing fuel gas over the anode surface, distributing oxygen over the cathode surface, and conducting current from the anode of one cell to the cathode of the next. Bipolar plates are typically formed of graphite or carbon-based composites which are conductive and have low density. Graphite, however, is brittle and has relatively low strength, making it difficult to handle. In addition, it is bulky, is expensive to machine, and is relatively porous. Metals are better electrical conductors than graphite, are more compact, are relatively easy to machine, and are usually not porous. However, the PEM fuel cell environment is very corrosive for all prior art materials but the noble metals, as water vapor, oxygen, and heat coexist in a PEM fuel cell. Furthermore, during long-term operation of a PEM fuel cell, small amounts of HF and H2SO4 are known to leach out of other components, leading to either pitting corrosion or formation of a high-resistance passivating layer on the cathode side of a metal bipolar plate. In the prior art, only conductive polymer or noble metal coatings are satisfactory solutions to this problem for metal bipolar plates, and each is expensive and requires an additional manufacturing step, which increases the overall cost of a PEM fuel cell.
Another example occurs in the electrical contacts formed in standard connectors having terminals formed of non-noble metals. Typically, the point of electrical contact inside a connector is a dimpled metallic surface pressed against another metallic surface, which may be flat or contoured. After the connector is assembled, the measured resistance across the connection interface is desirably less than 10 mΩ or 0.2×10−4 mΩcm2. Unfortunately, this initial value may change over time, because the metallic surfaces can oxidize, causing the measured resistance across the metallic interface to increase.
Copper, tin, silver, palladium, and hardened gold are all commonly used connector interface materials. Hardened gold provides the most reliable and stable interconnection, but even this material will degrade to a failure level due either to diffusion of an additive (cobalt or nickel) that hardens the gold to the surface or to wear-through of the gold layer and exposure of the more reactive base metal, typically a copper alloy. In all cases, the electrical interconnection will eventually fail.
What is needed is a simple and cost-effective means for providing and maintaining electrical conductivity of electrical contact surfaces in environments which produce high-resistance passivating or corrosion layers on base metal contact surfaces.
It is a principal object of the present invention to provide improved materials for forming or surface coating electrical contacts which are simple and inexpensive to manufacture and which maintain acceptable electrical conductivity of the surface during use.
It is a further object of the invention to increase the durability and reliability of electrical contacts and of apparatus equipped with electrical contacts.
It is a further object of the invention to reduce the manufacturing cost of apparatus incorporating electrical contact surfaces.